It is known in the art of luminescent screens used in radiographic applications and the like that the speed of such screens can be increased by addition of aminocoumarin brightening agents to conventional X-ray phosphors. It is also known that high speed screens can be prepared using a group of highly efficient X-ray phosphor compositions of the type Ba.sub.1.sub.-(x.sub.+y) Eu.sub.x Sr.sub.y FCl wherein Ba is responsible for the X-ray absorption and Eu is responsible for ultraviolet emission. Luminescent screens prepared from these phosphors exhibit increased stopping power accompanied by an unusual degree of actinic efficiency provided by the Eu activator. These screens also exhibit remarkable speed and conversion efficiency.
High actinic and conversion efficiency lead, however, to an undesirable effect known as quantum mottle. Cleare et al. in The Am. J. of Roent. and Rad. Physics, Vol. 88, No. 1, pp. 168-174 (July 1962) discuss the effect of quantum and screen structure mottle and suggest that quantum mottle may be minimized by lowering the effective conversion efficiency of the screen and the screen speed.
There is need for an X-ray screen in which mottle can be reduced at minimal sacrifice of screen speed.